Some would say that the old title was more descriptive or honest. The US Department of Defense, which is commonly known as the DoD for short, actually used to be titled more bluntly, “Department of War.”

The name change occurred in the late 1940s. With World War II over, the United Nations was taking steps towards what it hoped would be a lasting peace. In its Charter, the UN outlawed wars of aggression (wars which aren’t fought in defense), and as a result, top US military brass felt the American bureau needed a name, if only for PR reasons.
Above: The official Dept of War seal
So, from 1947 through 1949, Congress adopted a series of laws renaming (and reorganizing) the American national military establishment to a more politically correct naming scheme. Accordingly, the Secretary of War was renamed the Secretary of Defense. Perhaps only one vestige of the old naming scheme remains: the US Army War College in Pennsylvania.

Following suit, several other countries also renamed their war departments around the same time. For example, Great Britain similarly used to have a War Office, which was renamed to the Ministry of Defence in 1963.